In modern radios such as those used in cellular telephones, a high performance receiver path is used to detect signals below −110 dBm in the presence of blocking signals up to −20 dBm. This makes design of the radio and the frequency synthesizer challenging in terms of reducing power consumption. The receiver (Rx) path may also be used to sense the transmitter (Tx) signal for either envelope tracking or tuning the antenna. Furthermore, the receiver may be used to detect blockers or jamming signals to improve the quality of the wanted signal. This introduces practical implementation problems such as VCO (voltage control oscillator) pulling in the receiver.
In radio designs using full duplex (simultaneous Rx and Tx) modes of operation, such as those used in certain code division multiple access (CDMA) cellular telephone and next generation technologies, transmitted signals entering the receiver input can be an especially significant problem. In these technologies, an auxiliary Rx channel is sometimes used for power level sensing and balancing of both receiver and transmitter. These additional channels are known, and commercial ICs for implementing such auxiliary channels exist.
Consider the design of a conventional full duplex radio transceiver such as that depicted in FIG. 1. In such a design, a transmitter 100 output signal is passed to an antenna 104 through a switch 108. Since modern receiver designs may operate over multiple bands, multiple transmitters and receivers may be represented by transmitter 100 and receiver 112. Since the transmitter and receiver share the same antenna, for each band of frequencies used by the receiver, the design shown in FIG. 1 utilizes a separate filter 116, 120 through 124 (e.g., a SAW filter or the like) configured as notch filters. Switch 108 switches to the correct notch filter for the band of transmitted signals based on commands from a control processor 130 based on a selected channel or band. These notch filters are used to prevent the relatively high power from the RF amplifier 134 of transmitter 100 from entering the front end low noise amplifier of the receiver 112 and either damaging the receiver or degrading operation thereof.
As worldwide radio receivers are developed, as many as 20 (or possibly more) bands of frequencies may need to be accommodated to truly handle each possible frequency band.